Oxygen-Cluster-Modified Anatase with Graphene Leads to Efficient and Recyclable Photo-Catalytic Conversion of CO₂ to CH₄ Supported by the Positron Annihilation Study

Abstract

Anatase TiO₂ hollow nanoboxes were synthesized and combined with the graphene oxide to get nanocomposite of TiO₂/rGO (TG). Graphene oxide was used to modify the Oxygen-Clusters and bulk to surface defects. Anatase and TG composite were characterized with the positron annihilation, XPS, EPR, EIS and photocurrent response analysis. The relative affects of defects on the photocatalytic reduction (CO₂ to CH₄) were studied. The TG composites showed highest photo-catalytic activity after GO coupling (49 µmol g⁻¹ h⁻¹), 28.6 times higher photocurrent yields much higher quantum efficiency (3.17%@400 nm) when compared to the TiO₂ nanoboxes. The mechanism of enhanced photo-catalytic CO₂ conversion to CH₄ elucidated through electrochemical and photo-catalytic experiments with traceable isotope containing carbon dioxide (¹³CO₂). For the first time we discovered that diminishing the comparative concentration ratio of anatase from the bulk to surface defects could significantly increase the conversion of CO₂ to CH₄.

Introduction

The carbon dioxide (CO₂) emission is one of the serious environmental problems, therefore its reduction and sustainable conversion to fuels by using solar energy is highly desirable¹. Titania (TiO₂) is a chemically stable and biologically benign photo-catalyst, capable to convert harmful pollutants, particularly CO₂ reduction to fuel CH₄². Beside the well-known bulk defects, the surface defects are more sensitive to the photocatalytic activities. The surface defects in a pure crystal can act as e-h trap-sites along with the adsorption sites on the other hand the bulk defects act only as charge carrier traps where e-h are more likely to recombine. Therefore, there is great scientific importance to understand both the bulk and surface defect states. This study highlights the defect density effects on the photocatalytic activities, which may lead to the innovative design of photocatalysts. The defects in TiO₂ have been experimentally studied by the scanning tunneling microscopy (STM)³, electron paramagnetic resonance (EPR)⁴, time-resolved photoluminescence spectroscopy (PL)⁵ etc. These techniques allow understanding the effects of surface or subsurface defects, thereby helped to correlate the adsorption and surface reactivity.

The surface of graphene oxide consists of oxygen and hydroxyl functional groups capable to form new bonds with the TiO₂. When combined with graphene, the composite provides an effective approach to improve the photo-catalytic efficiency and light response. This occurs because graphene layers with 2D sp²-hybridized carbon atoms (pi-bond conjugated structure) have superior electronic properties and large specific surface area that significantly enhances the photo-catalytic properties^6,7. Compared with the other defect-probes techniques, positron annihilation lifetime spectroscopy (PALS) is a non-destructive technique with many advantages. For example, PALS is capable to probe not only surface but also bulk defects, sensitive to all kinds of defects particularly defects with complex structures such as open volume porousity, aggregates, voids, disclocations and grain boundaries. Therefore, PALS is a good addition to the XPS, EPR and PL techniques.

Herein, TiO₂ nano-boxes with bulk/surface defects were synthesized and characterized with the positron annihilation. We found that lowering the relative bulk to surface defects concentration ratio improved the photocatalysis efficiency. We report exceptionally enhanced photocatalytic CO₂ reduction capability of titania through defects and structural engineering, as well as magnetic titania hybrid for efficient recyclability. Simple routes were developed to synthesize the nano-crystalline anatase TiO₂ with hollow structure of nano-boxes. For the defect engineering and photocatalytic properties, nanocomposites of TiO₂/rGO (TG) and TiO₂-(Fe₃O₄@SiO₂)/rGO (TFSG) hybrids were synthesized through controlled microwave processing strategy. To improve the recyclability, TFSG samples with Fe₃O₄ magnetic nano-core and SiO₂ protecting shell were synthesized for the recovery of catalyst. Herein we discuss our findings in detail.

Results and Discussion

Structural characterization and surface composition

The XRD spectra of TiO₂, and TG composite (Supplementary Fig. 1a(i)) indicates that the samples have crystalline nature. The diffraction peaks indexed as (101), (004), (200), (105), (211), (204), (116), (220), (215) and (224) for the anatase phase matched well with the reference (JCPDS: 01-071-1166), indicating a single phase. The obtained TG sample shows TiO₂ peaks because rGO is not sensitive to the XRD (Supplementary Fig. 1a(ii))⁸. The Raman spectra of composites (Supplementary Fig. 1b) exhibit typical features of rGO with D-band at 1354 cm⁻¹ and G-band at 1611 cm⁻¹. The remaining four peaks from 100–700 cm⁻¹ belong to the TiO₂. When the geometric size of TiO₂ crystal approaches to nanometer range, the E_g Raman active mode blue-shifts to 150 cm⁻¹ (instead of 143 cm⁻¹) for the bulk phase⁹. The other specific vibration modes at 400 cm⁻¹ (B_1g), 523 cm⁻¹ (B_1g +A_1g) and 643 cm⁻¹ (E_g) are characteristic peaks of the anatase phase.

The C1s XPS spectra indicated the existence of chemical binding between TiO₂ and rGO (details in Supplementary information D1). Compared with the blank GO, the new peak at 285.5 eV is ascribed to the Ti-O-C bonding. Therefore there is covalent bonding between the TiO₂ and rGO. This is further confirmed by the significnt decrease in the FTIR peks at 454, 668 and 803 cm⁻¹. The bright-field TEM (Fig. 1a) of TiO₂ indicated hollow box like structures with narrow size distribution and good morphological uniformity. The TiO₂ nanoboxes on graphene sheets (Fig. 1b) showed well dispersed NPs on both the graphene sides. The reduced graphene oxide is transparent with wrinkles and hollow TiO₂ structures (HRTEM, lattice spacing of 0.35 nm, Fig. 1c), confirming the (101) planes of the anatase TiO₂. The SAED pattern (Fig. 1d) revealed a [010]-axis orientation, and the lateral plane of the of anatase corresponds to {010} facet that is vertical to the [010] orientation. The Fig. 1e STEM image shows hollow TiO₂ nanoboxes.

Figure 1

Characterization of TiO₂ hollow nanoboxes and TG composite. (a) Bright-field TEM images of the hollow TiO₂ nanoboxes. (b) TEM of TiO₂@rGO (TG composite). (c) The HRTEM shows the lattice spcing 0.35 nm (101). (d) corresponding SAED pattern. (e) STEM of hollow TiO₂.

Defect characterization by PALS, XPS and EPR

Previous reports showed oxygen defects on the surface of TiO₂ raise the local-states under the edge of the conduction band. Oxygen defects densities with one or two electrons are generated due to the missing oxygen atom in the bulk or on the surface, while the Carbon doped TiO₂ forms deep impurity states in the band gap^1,2,3,10,11. We observed (UV-vis spectrum, Supplementry Fig. 2) that bandgap of TiO₂ (3.2 eV) significantly decreased (2.6 eV) with increased absorption after hybridization with GO.

In this section, we used PALS (positron annihilation lifetime spectroscopy) to quantify the defect concentration obtained by the comparison of TiO₂ and TG composites. The Table 1 shows three positron lifetime components, τ₁, τ₂, and τ₃, with relative intensities I₁, I₂, and I₃ for the TiO₂ nanoboxes and TG composites. The shortest positron lifetime τ₁ (0.1985 ns) can be assigned to the bulk defects with free annihilation of positrons inside defect-free crystal¹². Where as the longer lived positrons τ₂ (0.3937 ns) can be attributed to the surface defects originating from the positrons trapped by the larger defects such as oxygen vacancies or clusters, homodimers, trimers or larger¹⁰. The second positron lifetime components (τ₂) for the TiO₂ nanoboxes and TG composites are much higher than their respective first positron lifetime components (τ₁). The variation of τ₂ value from 0.3937 ns to 0.3864 ns was evident that the TiO₂ nanoboxes contain an abundance of surface oxygen defects and compressive bonding but reduced to atomic dimensions in the TG composites. The largest component (τ₃) is assigned to the micropores and to the annihilation of orthopositronium atoms¹¹. Besides the lifetime factor of the positron, its relative intensity (I) gives information regarding the relative concentration of the defects¹³. The coupling of graphene oxide with TiO₂ leads to the decreased I₁/I₂ (1.23 to 1.03), thereby clearly indicates the relative bulk to surface defects concentration decrease.

Table 1 Positron life time and relative intensities of the TiO₂ and TG sample

The samples were further investigated by the XPS to get information about the chemical composition, oxygen vacancies and Ti oxidation-state inside TiO₂ nano-boxes after microwave treatment with the rGO. The O1s spectrum of TiO₂ nanobox (Fig. 2a) shows a peak at the binding energy of 529.3 eV, corresponding to the Ti(IV) bound oxygen (O²⁻). The peak at 531.6 eV can be attributed to the high binding energy component originated from the loss of oxygen (oxygen vacancies)¹⁴. The peak at 532.9 eV (low binding energy component), is due to the adsorption of HO⁻ on the surface. The O1s spectrum of TG composite (Fig. 2c) showed a strong O²⁻ peak at 529.5 eV. The second peak related to the oxygen vacancies O* (531.6 eV), showed significant suppression after rGO hybridization (microwave treatment). A slight shift (0.2 eV) in the peak position (529.5 eV) for the oxygen anions (Ti–O–Ti) is consistent with the Ti 2p core-level due to hybridization with graphene. Furthermore, inside TG sample the third low energy peak (near 532.9 eV) related to the surface hydroxyl group almost vanished. These observations confirmed that the concentration of surface defects is related with both surface hydroxyl groups and oxygen vacancies were significantly lowered in the hybrid sample¹⁵. The Fig. 2b,d schematically illustrates oxygen vacancies before and after GO hybridization.

Figure 2

(a) XPS spectrum (TiO₂) of measured O1s spectrum, (b) simulated oxygen vacancies in 2*2-anatase crystal of the TiO₂ nanoboxes. (c) Measured XPS O1s spectrum of the TG composites. (d) Healing of the oxygen vacancies in the 2*2-anatase crystal.

The defects were further analyzed by the EPR (Fig. 3) because of the sensitivity of EPR towards the different defects, caused by the lower or extra electrons present on different atoms (O⁻, O²⁻, Ti³⁺, Ti⁴⁺) under different surrounding atoms (spin Hamiltonian, perturbation theory). The EPR spectra for the TiO₂ shows several peaks beside the main peak (g = 2.05). This observation clearly indicates presence of several types of defects. The previously reported EPR spectra indicated that the initial hole-trapper at the lattice of oxide ions at hydrated TiO₂ surfaces are the hydroxyl radicals. These trapped holes are the defect centers termed as bridging O⁻ species¹⁶. The presence of signal at g = 2.05 with broad underline peak in TiO₂ nanobox (Fig. 3a) represents the existence of significant oxygen vacancies¹⁷. This occurs because electrons trapped at Ti⁴⁺ sites can form Ti³⁺ (XPS, Fig. 2) and holes at subsurface oxide ions can form O⁻¹⁸. The obtained EPR spectrum for the TG composites (Fig. 3b) shows a strong paramagnetic signal at g = 2.00 with loss of extra sharp-peaks. This represents the healing of oxygen vacancies (smoother spectrum) and other defects. Therefore we concluded that rGO hybridization has remarkably lowered the oxygen vacancies and other defects. These observations were further tested by the hydrofluoric acid (HF) addition experiments. Defects were introduced into TiO₂ by changing the amount of HF (details in Supplementary Information D2). The comparison showed defect introduction by the HF (0, 50 µL) changed the shape of EPR, introduced extra peaks (less smooth EPR) and modified the g value of TiO₂.

Figure 3

(a) EPR spectra of the TiO₂ and (b) TG composite at room temperature.

Absorption

The UV-Vis spectra of TiO₂ nano-boxes and composites (Supplementary Fig. 2) exhibit extended absorption therefore indicates lowered band gap of composites when compared with the standard TiO₂. The decrease in band gap has been attributed to the formation of C-Ti bond¹⁹. It is well-known that the extended absorption of light for the TG composite opens the possibility of higher photo-catalytic activity of a given photo-chemical reaction. Furthermore, considering the practical applications, the recyclability of photo-catalyst is another challenge to avoid contamination and higher economic efficiency. The photo-catalyst composites containing magnetite can easily separate from the solution by an external magnet. Therefore, the designing of efficient, magnetic photo-catalytic composite with reliable and stable structure has high importance. The non-toxic SiO₂ coating layer, conjugates well with functional groups, and known to protect Fe₃O₄ from oxidation thereby increases stability of NP-decorated photo-catalyst. Therefore, we have incorporated these materials to construct our photo-catalysts (TiO₂-Fe₃O₄@SiO₂)@graphene TFSG composite. The percentage of nanocomposites was TiO₂ (85%), Fe₃O₄@SiO₂(5%) and graphene (10%). The XRD spectra of TiO₂, Fe₃O₄@SiO₂ and (TiO₂-Fe₃O₄@SiO₂)@graphene TFSG composite (Supplementary Fig. 3) indicates that all the samples are crystalline in nature. The obtained TFSG sample showed both peaks of TiO₂ and Fe₃O₄@SiO₂ nanoparticles (Supplementary Fig. 3(iii)), thus confirming the composite nature. The HRTEM of composites showed uniformely coated Fe₃O₄ with SiO₂ and and TiO₂ (Supplementary Fig. 4a,b).

The comparison of magnetic hysteresis loops related to the Fe₃O₄, Fe₃O₄@SiO₂ and TFSG composites (Supplementary Fig. 5) showed room-temperature ferromagnetism for the Fe₃O₄ and Fe₃O₄@SiO₂ and TFSG composites. The results show that there is a decrease in the magnetic saturation of TFSG composite due to the non-magnetic TiO₂. The saturation magnetization value (M_s) of the TFSG composites is 3.18 emu g⁻¹, which is lower than those of the Fe₃O₄@SiO₂ (57.48 emu. g⁻¹) and Fe₃O₄ (64.03 emu. g⁻¹). This value of magnetization is good for the efficient recovery of photo-catalyst.

Photocurrent and charge separation

The transient photocurrent response for the TiO₂, TG and TFSG composites were compared (Fig. 4a). The photocurrent (photo-electrochemical) response of the TiO₂ nanobox was weak (8.2 × 10⁻⁵ mA.cm⁻²). While the TG composite showed significantly increased photocurrent response 2.344 × 10⁻² mA.cm⁻² (28.6 times higher) and 9.786 × 10⁻⁴ mA.cm⁻² for the TFSG composites (11.96 times higher). This significant photo-current enhancement attributed to the lower recombination rate of electron–hole pairs and longer lifetime due to the presence of rGO sheets (2D 𝜋-𝜋 conjugation structure) as compared to the TiO₂. The comparison of EIS curves (Fig. 4b) showed the single-semicircles (Nyquist plots) in the high frequency range and a linear behavior in the low frequency region. The lowest charge transfer resistance (R_ct) was observed (4.9 Ω) for the TG composite when compared to the TFSG (6.2 Ω), and TiO₂ (6.5 Ω). Therefore, the TG composite exhibits the lowest R_ct, indicating lower charge-transfer resistance resulting in much faster separation of photo-generated species^20,21.

Figure 4

(a) I–t curves for the photocurrent responses of the TiO₂ nanoboxes (a), TG (b) and TFSG composites (c). (b) The Nyquist plot for the TiO₂ nanoboxes (a), TG (b) and TFSG composites (c).

To verify the carbon detected in CO₂ conversion originates from the decomposition of photo-catalyst or from the added CO₂ gas, we carried out isotopic experiments by using traceable ¹³CO₂ under identical photo-catalytic reaction conditions. The products of photo-catalytic reaction were identified and quantified by the GC-MS (Supplementary Fig. 7). The ¹³C, ¹³CH, ¹³CH₂, ¹³CH₃ and ¹³CH₄ molecular fragments were detected. These observations from MS spectrum confirmed that the produced CH₄ indeed originated-from the reduction of CO₂ gas and not from the photocatalyst decomposition.

Photo-catalytic activities

The photo-catalytic activities of TiO₂ nanoboxes, TG composites and TFSG composites samples were evaluated for CO₂ conversion with triethanol amine as the electrons donor (Fig. 5a). The TiO₂ nanoboxes showed weak response for the photo-catalytic CO₂ conversion to CH₄ (~15.0 µmol g⁻¹ h⁻¹) after 1 h irradiation. The TG composites demonstrate the highest (~49.0 µmol g⁻¹ h⁻¹) activity for CO₂ conversion. Since, the TFSG composites were hybridized with magnetic nanoparticles (Fe₃O₄@SiO₂) for easier recovery thereby it showed lower CO₂ conversion to CH₄ activity (27.0 µmol g⁻¹ h⁻¹) than TG, but still 3.2 times higher than that of TiO₂ nanoboxes (15.0 µmol g⁻¹ h⁻¹). The main detected gas products were CH₄, H₂ and CO (GC analysis). The Fig. 5b shows the quantitative comparison of produced H₂ during this experiment. We further performed blank tests to confirm that CH₄ comes from the reduction of CO₂. After blank photo-catalytic experiment without CO₂ under the same conditions, we have not detected any CH₄.

Figure 5

(a) Photocatalytic conversion of CO₂ to CH₄ after 1 h, (b) H₂ production by the photocatalysis (UV-Vis irradiation) of TiO₂ nanoboxes, TG composites and TFSG composites.

The Fig. 6 shows comparison of time dependent H₂ evolution for the TiO₂, TG and TFSG composites. This water splitting reaction showed a sustained H₂ release rate of ~18.46 µmol/h for the TG composites. The standard solar light illumination for 8 hours, the hydrogen evolution kept increasing, demonstrating the robust photo-catalytic performance. This observation indicates stable and active photo-catalyst for long term water splitting. After conversion of CO₂ to CH₄ (5 h), samples were collected, washed several times and dried. The TEM results (Details in Supplementary information D3) showed stable composite after 5 h light irradiation.

Figure 6

Time courses of H₂ evolution resulting from the reaction of water by TiO₂ nanoboxes, TG composites and TFSG composites irradiated with UV-visible light.

The rate of H₂ evolution followed the following order: TG composites > TFSG composites > TiO₂ nano-boxes respectively. Quantum efficacies of these samples were calculated (Supplementary information 8) as 3.02%, 2.01%, and 0.3%, respectively for the TG, TFSG, and TiO₂ nanobox. In the CO₂ reduction, the photo-generated electrons-holes would migrate to the rGO surface. The conduction band of TiO₂ nano-boxes and the work function of rGO are reported as −4.2 eV and −4.42 eV, respectively^22,23. The proximity of energy levels is helpful for the photo-generated species transfer from the TiO₂ nano-boxes conduction band to the rGO thereby produces radicals. The reason behind is the adsorption kinetics of the CO₂ (10⁻⁸ to 10⁻³ s) on TiO₂ nanoboxes is slower than the e-h recombination time (10⁻⁹ s)^24,25. The composites increased the lifetime of the charged species that further improved the selective formation of CH₄ gas along with other minor product gases. The photo-catalytic performance supports our proposed interpretations of the CO₂ conversion to CH₄ through reduction mechanism.

As reported the hydroxyl radicals (·OH) are recognized to be important intermediates for CH₄ conversion. The mechanism of charge separation, coumarin fluorescence method (coumarin + ^•OH → fluorescent 7-hydroxycoumarin) is used to detect the amount of hydroxyl radicals (^•OH) produced via photochemistry. The procedure relies on the basic principle that: the stronger the fluorescence signal being observed, the larger the ^•OH produced. We further compared the produced ^•OH for TiO₂ nanoboxes, TFSG and TG composites. The amount of produced ^•OH in TG and TFSG is larger than the TiO₂ nanoboxes (Fig. 7), with TG composites showing the highest fluorescence intensity. Hence, the TG composites produced highest amount of ^•OH thereby confirming the excellent photo-electrochemical and photo-catalytic properties.

Figure 7

The ^•OH radical amount-related fluorescence spectra of (a) TiO₂ nanoboxes, (b) TFSG and (c). TG composites.

In summary, we have explored the photo-catalytic reduction of CO₂ on TiO₂ nano-boxes, TiO₂ hybridzed with graphene (TG) and TiO₂-(Fe₃O₄@SiO₂)-rGO nanocomposite (TFSG). Oxygen clusters along with surface/bulk defects in TiO₂ nanoboxes play vital role in efficient photocatalysis. Positron annihilation results revealed decreased Oxygen clusters (mono, dimers, trimers) and the relative concentration ratio of bulk/surface defects in TiO₂ nanoboxes efficiently enhanced the photocatalytic mechanism. The photo-catalytic experiments and traceable isotopes containing carbon dioxide (¹³CO₂) analyzed through mass spectroscopy confirmed the source of CH₄ production. The TG composite exhibits 3.2 times (~49.0 µmol g⁻¹ h⁻¹) higher photo-catalytic reduction of CO₂ to CH₄, when compared to the TiO₂ nanoboxes. The TFSG composites retained magnetization, therby provides an easy way for the catalyst recyclability. The XPS study revealed that number of oxygen vacancies significantly decreased after GO hybridization, further confirmed by the EPR result. The excellent electrochemical and photo-catalytic properties were assigned to the lowered defects, proximity of energy levels, much higher photo-current and higher quantum efficiency (3.17% @400 nm). We have presented 4 important evidences (positron annihilation (I₁/I₂ lowered), XPS (peaks 532, 531 eV decreased), EPR (smoothed, enhanced-signals), EIS (lowered charge-transfer resistance R_CT) confirming that the GO modified the TiO₂ to get appropriate photo-catalytic properties. Further by comparison and appreciation with the recent research^{26,27,28,29,30,31} we believe, this research highlights the importance of meticulous design of the photo-catalysts required to improve the selective conversion of CO₂ towards valuable fuels³².